U.S. Pat. No. 3,738,116 entitled "COMPRESSOR UNLOADER INDICATOR AND REFRIGERATION SYSTEM CONTROLLED THEREBY" to Edward S. Gazda and assigned to the common assignee, discloses an unloader indicator and control mechanism associated with a typical helical screw rotary compressor. The unloader indicator of that patent utilizes a rod reciprocating with a slide valve and positioned within a fixed concentric, non-magnetic tube. The rod carries a permanent magnet, and the concentric tube supports a magnetic follower which is slidably mounted on the tube in concentric fashion for free sliding movement axially on the same but constrained to follow the magnet due to the magnetic attraction between the follower and the permanent magnet fixedly mounted to the reciprocating rod. The position of the follower, therefore, is indicative of the position of the slide valve. The indicator and control system utilizes an outer fixed tube which concentrically surrounds but is spaced from the magnetic follower. This tube may be transparent to allow viewing of the follower such that an indicator scale carried by the outer tube clearly identifies the exact axial position of the follower and thus the unloader slide valve. Further, a pluralty of reed switches are mounted to or incorporated within the outer tube at spaced longitudinal positions on that tube with the reed switches changing state when the permanent magnet passes by the switches. In turn, the reed switches are electrically connected via holding relays to the solenoid operated valves associated with the hot gas bypass circuit and the liquid refrigerant feed from the superheater to the refrigeration system chiller evaporator (as an example) for controlling operation of the flow of the working fluid, in this case refrigerant, through the system and to and from the compressor.
While the unloader indicator and the refrigeration system control components forming a part of that assembly function adequately, the unloader is somewhat complicated, and there is no direct mechanical coupling between the slide valve itself and the actuator elements for effecting the change of state of the switches, raising hysteresis problems.
It is, therefore, a primary object of the present invention to provide an improved unloader indicator for a helical screw rotary compressor which is of simplified construction, provides a positive drive between the slide valve and the actuators for the screw compressor control system, and wherein an analog electrical output signal, representing displacement of the slide valve, is readily effected.